Primary use is made today of materials known as “cream solders” for electron packaging applications (for example, see Patent Document 1). Cream solders are compositions which include solder particles, a fluxing component and a solvent. When a cream solder is heated in a reflow oven, the solder particles melt at the melting point and above. At an elevated temperature, the fluxing component removes the oxide film (oxide layer) at the surface of the solder particles, allowing the solder particles to coalesce and thus bringing the component mounting operation to completion. The result is a highly productive process capable of connecting numerous components at one time. Examples of fluxing components which are added include rosin constituents such as abietic acid, various amines and their salts, and high-melting organic acids such as sebacic acid and adipic acid.
Lead eutectic solder, which is a typical conventional solder, has a melting point of 183° C. However, the Sn—Ag—Cu solders typical of the lead-free solders that are starting to be used in keeping with the current trend toward the phasing out of lead, have melting points that are about 30° C. higher. Hence, in prior-art solder reflow processes, electronic packaging is carried out at elevated temperatures of up to 215 to 260° C.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-185884
When electronic circuits which include components incapable of withstanding elevated temperatures of 215 to 260° C. are packaged, it is necessary to carry out a separate step in which only these components are mounted by carrying out spot soldering or are mounted using a silver paste or the like. Unfortunately, this has greatly lowered the productivity.
Conductive pastes which employ, as solder particles having a melting point lower than 215° C., the alloy Sn42/Bi58 (melting point, 139° C.) are known. However, the following problems are associated with the use of this low-melting solder.
(1) Because the above low-melting solder is relatively brittle compared to lead eutectic solders and Sn—Ag—Cu solders, it leaves something to be desired in terms of strength and toughness. When a component is attached by only a solder connection, there is a tendency for the component to fall off and for cracks to arise in the solder connection due to temperature cycling and shock.
(2) Conventional fluxing components dissociate at a high temperature and exhibit a strong chemical activity against metal oxides. However, under low-temperature reflow conditions such as the above, an effective fluxing action does not arise, making it difficult for the solder particles to coalesce even on melting.
One conceivable way to, resolve above problem (1) in particular is to use a thermosetting resin as the binder, disperse low-melting solder particles in this binder and carry out solder bonding, thereby having the component attached not only by the solder connection but also by the cured resin. Although this would be expected to provide large improvements in strength and toughness, there are no known and effective fluxing components that can be made to coexist during such a process.